The dependence of perceived brightness on flash luminance and duration was determined for dark-adapted observers, with target size, retinal location, and wavelength varied parametrically. In the first series of experiments, observers made magnitude estimations of the brightness of flashes of varying luminance and duration. Perceived brightness varied as a power function of luminance, with simple fractional exponents: 1, (1/2), (1/3). The exponents for brightness depend upon both target size and flash duration. The second series of experiments determined how the Broca–Sulzer brightness enhancement shifts to briefer or longer durations with changes of luminance. Observers adjusted the duration of constant-luminance flashes to produce a maximally bright flash. The flash duration producing maximum brightness varied as a power function of luminance with simple fractional exponents: −(1/2) for point sources and −(1/3) for extended sources. The regularity of the exponents suggests that a simple mechanism underlies the encoding of brightness information in the dark-adapted state.
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